There are many processes where it is necessary to bring into a contact a fluid and solid particulate matter. Frequently, chemical reactions as well as physical phenomenon take place as a result of such contact. An example of a process in which the present invention may be applied is the treatment of a gas stream to remove a component by means of contact with particulate solids, such as removal of sulfur dioxide from a flue gas strea upon passing the flue gas through a bed comprising a sulfur oxide acceptor such as copper-bearing alumina spheres.
However, the present invention was developed primarily for use with particles of hydrocarbon conversion catalyst in a moving bed. Further discussion and explanation of the invention will deal with applications in hydrocarbon conversion processes, though it is not intended that the practice of this invention be limited to such use.
In many modern hydrocarbon conversion processes, a hydrocarbon in vapor form is contacted with catalyst particles which are confined within a reaction zone comprised of a vertically-elongated moving bed of catalyst particles. The particles are confined in the bed by means of catalyst retention screens, through which catalyst cannot pass. Catalyst in the bed moves downward under the influence of gravity. Hydrocarbon vapor passes through the catalyst bed in a direction which is substantially transverse to the direction of catalyst movement. Bed thickness in the direction of gas flow is uniform. U.S. Pat. No. 3,706,536 (cited above) may be consulted for information on such reactor systems.
The present invention is also useful for effecting treatment needed to regenerate spent hydrocarbon conversion catalyst. After a period of time in use, the catalyst used in a hydrocarbon conversion process must be regenerated, that is, it must be treated to restore it to a satisfactory level of activity and stability for catalyzing the reaction. Regeneration consists of several different processing steps. Usually, the first step is contacting the catalyst particles, in a vertically-elongated movable bed, with a hot oxygen-containing gas stream in order to remove, by means of combustion, coke which accumulated on the catalyst particles while they were used in the hydrocarbon conversion zone. Coke is comprised primarily of carbon, which is converted to carbon dioxide in a combustion zone, or burn zone. The burn zone is located within a vessel which is commonly called a regenerator, or regeneration vessel. U.S. Pat. No. 3,652,231 (cited above) may be consulted for informatuon on hydrocarbon conversion catalyst regeneration.
Though the present invention is useful in both reaction zones and regeneration zones which contain light catalyst, it will be explained by means of a detailed discussion of catalyst regeneration.
Catalyst particles are often comprised of one or more Group VIII noble metals (e.g. platinum, iridium, rhodium, palladium) and a halogen combined with a porous carrier, such as a refractory inorganic oxide. Alumina is a commonly used carrier. The halogen is normally chlorine. The particles are usually spheroidal and have a diameter of from about 1/16 to about 1/8 inch (1.5-3.1 mm), though they may be as large as 1/4 inch (6.35 mm). In a particular regenerator, however, it is desirable to use catalyst particles which fall in a relatively narrow size range.
Movement of catalyst through a regeneration zone and a reaction zone is often referred to as continuous, though in practice, it is semi-continuous. By semi-continuous movement is meant the repeated transfer of relatively small amounts of catalyst at closely spaced points in time. For example, one batch per minute may be withdrawn from the bottom of a zone and withdrawal may take one-half minute, that is, catalyst will flow for one-half minute. If the inventory in the reaction zone is large, the catalyst bed may be considered to be continuously moving.